les is right about needing info but in this case oak is what it is.
it may have been exposed to a form of rot in the past that has reduced the structural integrity of the timber making it easier to cut and lighter but not necessarily causing any spalting.
also don't forget that the average weight of oak is exactly that, an average. you will get bits alot heavier and alot lighter. some species i have encountered have had a 10lbft3 difference from the average weight of that species which really threw me when i was identifying it as i presumed it should have weighed more.
don't forget that the wood at the top of the tree weighs less than the wood at the base as you don't want heavy wood 100' up in the air.
lastly you don't know exactly which species of quercus you have there not to mention the growing conditions of the plant itself all of which can greatly affect the weight and specifics of the material the tree produces.
pictures 1&2 are oak, 3&4 are yew...